I once had that same problem with my '88 260e. It started as an occasional stall and became much worse over the course of several months. The problem turned out to be the airflow sensor pot mounted on the front of the airflow sensor. It wears out with time and causes the problem you describe, especially at idle and when coasting to a stop. Try a search on "airflow sensor pot".

The Bosch part is available from Olyparts.com. It is kind of sensitive to replace but not horribly difficult.

There are other causes of stalling that should be investigated before assuming the pot is bad.

John